Piezoelectric crystal mounting



Dec. 3, 1946. c. BALDWIN 2,412,030 PIEZOELECTRIC CRYSTAL MOUNTING Filed oct. 27. 1938 Invehtob: Charles F Baldwin,

Hus Attorney.

Patented Dec. 3, 1 946 PIEZOE LECTRIC CRYSTAL MOUNTING Charles F. Baldwin, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application October 27, 1938, Serial No. 237,237 28 Claims. (cl. 171-327) My invention relates to piezo-electric devices and more particularly to supporting means for a piezo-electric device. While not limited thereto, my invention is particularly useful for supporting a high frequency piezo-electric device which normally is operated at one of its harmonic frequencies. By harmonic frequency, I mean a frequency which is substantially a harmonic of the crystal fundamental frequency, sometimes referred to as a mechanical harmonic frequency of the crystal, I

An object of my invention is to provide a new and improved method of supporting a piezo-electric crystal with the active parts of its oscillating surfaces out of physical contact with fixed physical objects.

A further object of my invention is to provide a greatly simplified and inexpensive supporting arrangement for a pieZo-electric crystal and one having the advantages of extreme compactness, sturdiness and a high degree of reliability.

Another object of my invention is to provide a crystal mounting in which the crystal is freely supported in, and has each of its surfaces in close heat transfer relation with, a fluid cooling medium such as helium, argon or the like.

A further object of my invention is to provide a new and improved Diezo-electric crystal of the high frequency type. By high frequency, I'mean any frequency from about one megacycle to the highest frequency at which crystals may be used. My invention contemplates that the electrical circuit terminals and physical supporting means for the crystal are fixedly secured to the crystal by securing devices which extend through apertures provided inthe body of the crystal.

The novel features which I believe to be characteristic of my invention are pointed out with particularity in the appended claims. My invention itself, however, -will be better understood by reference to the following description taken in connection with the accompanying drawing in which Fig. 1 represents an embodiment of my invention; Fig. 2 is a bottom view of a crystal holder constructed according to my invention; Fig. 3 illustrates an additional view of the crystal at a right angle to the view in Fig. 1 and Figs. 4 and 5 show certain modifications of my invention.

Referring particularly to Fig. 1 of the drawing, my invention is illustrated as embodied in a crystal holder having an outer housing Ill of metal, glass, or the like in the interior of which is supported. as by a bracket H, a piezo-electric crystal l2. The crystal I2 is here shown merely for purposes of illustration as greatly exaggerated in size. In practice, a crystal which is ground for fundamental operation has a definite relation between the length of its principal frequency determining dimension, measured in thousandths of an inch, and its operating frequency, measured in megacycles, which relation is a product between about 70 and about 113, depending on the particular manner in which the crystal is cut. Crystals operated at mechanical harmonics, as referred to above, also have such a product which is usually over 200. A particular type of such an harmonically operated crystal, which is especially useful, operates at its third mechanical harmonic, and has a product of about 300 of its operating frequency in megacycles and length of its principal frequency determining dimension in thousandths of an inch.

The piezo-electric crystal, here 'shown as of the harmonically operated type, is provided with flexible metallic electrodes l3, M (Figs. 1 and 3) fixedly secured in opposing relation over the central portion of each of the crystal faces. The electrodes I3, It, as shown in Fig. l, are very small relative to the size of the crystal, thereby to have small mass and minimum interelectrode capacity to enhance operation of the crystal at a harmonic frequency. The electrodes are preferablyevaporated upon the faces of the crystal to assure minimum mass and are placed only on the central portion of each face to assure minimum capacity. In the present embodiment, each of the electrodes l3, l4 has an extended portion i5, i5,

respectively, which extends to opposite corners of the crystal body.

Such a crystal of -the harmonically operated type as described has certain relatively mechanically inactive surface portions lying between the central portion of the crystal and its periphery. These inactive portions may extend inward from the periphery of the crystal by substantial distances, and may be clamped with forces of several hundred pounds without interfering with the operation of the crystal. It is highly advantageous to clamp crystal supports or electric circuit ter minals for the electrodes to such mechanically inactive surface portions by suitable means.

Electrical circuit connecting terminals l1, l8, are fixedly secured by small machine screws [9, 20 to the bodyof the crystal in electrical connection with the respective extended portions l5, l6 of the crystal electrodes. In the preferred embodiment of my invention, the machine screws I9, 20 extend through holes 28 (Fig. 3) drilled through the body of the crystal. These holes operating characteristics.

through which the machine screws are passed lie bracket i i and through a hole drilled in the lower edge of thecrystal near the periphery thereof where, as explained above, the inactive surface portions are located. The bracket it may be shouldered, if desired, at the point where it contacts the edge of the crystal, the shoulder extending under the lower edge of the crystal to prevent the crystal turning about the screw 2!. The bracket H is welded to or otherwise fastened in position on a base plate or header 22 which forms an end wall of the housing iii.

The housing ill is provided at its lower end with a base 23 similar to those used on vacuum tubes, the base having electrical connecting prongs 24 suitably arranged for insertion into a standard vacuum tube socket. The use of a tube base and a tube socket in this manner greatly facilitates the replacement of one crystal unit with another having the same or diiferent Electrical connection is made between the several terminals 2d and the electrical terminals ll, 08 by electrical conductors 2d, 26, which extend through insulating beads 25, it, provided in the header 22.

The interior of the housing It may be evacuated through the sealing-d stem 2i which extends through and is hermetically sealed to the header 22. After evacuation, the housing may be refilled to a desired pressure, usually slightly less than atmospheric, with a dry gaseous fluid medium as air or, if greater heat conductivity is required, with helium, argon, or the like. After the refilling operation, the housing iii is hermetically sealed by closing the end of the stern It will, of course, be understood that the header 22 is hermetically sealed to the housing ill. The use of a dry gas in the interior of the 4g support the weight of the crystal, crystal elec= trodes and the electrical terminals. "This modification has the important advantage that the conductors 25, which have a certain amount of resiliency, support the crystal in resilient manner from the header 22. i

A modified support arrangement is illustrated in Fi 5, The support ii is here provided with an integrally formed stud 29 which extends through the aperture 28 provided in the crystal 8? and is threaded at its end to receive a nut 30. A washer 35, preferably formed of a soft metal, as Woods metal, prevents damage to the surface of the crystal when the nut 36 is tightened rigidly to secure the support it to the crystal.

It will now be apparent that I have provided a small, compact, crystal unit of sturdy construction in which the crystal is enclosed in a hermetically sealed housing. The housing when con= structed of metal provides an electrostatic shielding of the crystal in addition to protecting the crystal from the deleterious effects of dirt and moisture. It will be evident that a crystal and its holder constructed in accordance with my invention has the added advantage that the crystal is supported in the best possible heat transfer relation to the surrounding cooling medium.

Thus, while I have illustrated a particular em= bodiment of my invention, I do not wish to be limited thereto since many modifications may be made in the several elements employed and in their arrangement and I, therefore, contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope oi my invention.

What I claim as new and desire to secure by Letters Patent 01' the United States is:

l. A piezo-electric crystal whose operating ire= and greater than a megacycle. said crystal having a an aperture formed therethrough near the pchousing is desirable in that it protects the crystal from dirt and moisture. Helium, argon, or the like gas, additionally aids in the rapid transfer of heat from the crystal-to the atmosphere surrounding the crystal housing. It should be shows the arrangement of the base prongs 26, 2c

and the manner in which the cross-sectional view 7 of Fig. 1- was made along the plane i-l.

Fig. 3 is another view which shows more clearly the arrangement of the crystal l2, the electrodes l3 and i l, the securing screws it, 28, and the supporting bracket l i.

A modification of my invention is shown in Fig. 4 in which elements corresponding to like elements of Fig. 1 are designated by like reference characters. The supporting bracket ii of the Fig. 1 arrangement is eliminated in this modification and the crystal i2 is supported directly by the electrical conductors 25, 2B. The electrical conductors 26, 25 are of larger cross-section than are the same conductors of the Fig. 1 arrangement for purposes of mechanical strength since it is apparent that the conductors must,

riphery thereof, and supporting means extending through said aperture and secured to said crystal in rigid supporting relation therewith.

2. A piezo-electric crystal of the harmonically operated type having a relatively active central face portion and a relatively inactive peripheral face portion, electrodes fixedly secured in opposing relation to each other over said active portions, electrical circuit terminals, and means extending through apertures formed in said peripheral portions for fixedly securing said terminals to said crystal each in electrical connection wit a respective one of said electrodes.

3. A piezo electric crystal having flexible electrode coatings in opposing relation over the central portion of opposite faces thereof, said coatings being electrically conductive and each having a current conducting portion extending to respective spaced apart points near the periphery tal frequency and is also greater than a megacycle, of means extending through apertures formed through said crystal for securing electrical circuit terminals to said crystal, electrodes evaporated on opposite faces of said crystal. means for electrically connecting each of said electrodes with a respective terminal, "and meansutilizing said terminals for supporting said crystal suspended free of physical objects.

5. In a piezo-electric crystal mounting, the

' extending through an aperture formed in the peripheral edge of said crystal for fixedly securing said bracket to said crystal.

6. A piezo-electric crystal whose operating frequency is higher than its fundamental frequency and higher than a megacycle, said crystal having electrodes positioned adjacent opposite faces thereof, and means passing through a hole through said crystal for pressing an electrical terminal against said crystal, said means being arranged to maintain electrical connection lee-- tween said terminal and one of said electrodes.

'7. In a support for a piezo-electric device, the combination with a piezo-electric crystal whose operating frequency is higher than its fundamental frequency and higher than a megacycle, of means passing through an aperture formed through the periphery of said crystal for pressing an electric circuit terminal with substantial force against said crystal, electrodes on opposite faces of said crystal, said means maintaining electrical connection between said terminal and one of said electrodes, and means utilizing said terminal for supporting said crystal free of surrounding objects.

8. A piezo-electric crystal whose operating frequency is a mechanical harmonic frequency of said crystal higher than its fundamental frequency, conducting material evaporated in a thin film upon the central portions of opposite faces of said crystal to form electrodes therefor, said thin films extending to respective spaced apart points near the periphery of said crystal, and means passing through a hole through said crystal at each of said spaced points for pressing an electric circuit terminal against the respective thin film to maintain electric connection between said terminal and the respective one of said electrodes. U

9. In a piece-electric device, a piece-electric crystal having an operating frequency higher than its fundamental frequency, said crystalhaving relatively mechanically inactive surface portions,

an electrode positioned adjacent said crystal, an

electric circuit terminal, means for clamping said terminal to one of said inactive surface portions of said crystal with force sufficient to maintain said terminal in fixed relation to said crystal, and means for connecting said terminal to said electrode.

10. In a piezo-electric device, a piezo-electric crystal having an operating frequency which is a mechanical harmonic frequency of said crystal higher than its fundamental frequency, a thin film of conducting material evaporated only upon the central portions of opposite faces of said crystal to form electrodes therefor, said thin films extending to respective spaced apart points near the periphery of said crystal, a pair of electric circuit terminals, and means for clamping said electric circuit terminals to the respective thin film portions at each of said spaced points to maintain electric connections between said terminals and the respective electrodes.

11. In a piezoelectric device, a piezo-electric crystal having an operating frequency which is a mechanical harmonic frequency of said crystal higher than its fundamental frequency, said crystal having relatively mechanically inactive surface portions, a thin film of conducting material evaporated on a surface of said crystal forming an electrode and having a portion extending over one of said inactive portions, an electric circuit terminal, and means for clamping said terminal to said thin film over said inactive surface portion to maintain electric connection between sald terminal and film.

12. In a piezo-electric. device, a piezo-electric crystal having an operating frequency Which'ls a mechanical harmonic frequency of said crystal higher than its fundamental frequency, said crystal having relatively mechanically inactive surface portions, a thin film of conducting material evaporated on a surface of said crystal and forming an electrode thereon, said thin film having a portion extending over one of said inactive surface portions, an electric circuit terminal, means ill extending over one of said inactive surface 1301! for clamping said terminal to said thin film over said inactive surface portion to maintain electric connection between said terminal and film, and means for supporting said crystal from at least one of said inactive portions.

14. In a piezo-electric device, a piezo-electric crystal having an. operating frequency which is a mechanical harmonic frequency of said crystal higher than its fundamental frequency, said crystal having relatively mechanically inactive surface portions, a thin film of conducting material evaporated on a surface of said crystal and forming an electrode, said thin film having a portion tions, an electric circuit terminal, means for clamping said terminal to said thin film over said inactive surface mrtion to maintain electric connection between said terminal and film, and means utilizing said terminal for supporting said crystal.

15. In a piano-electric device, a piezo-efectric crystal having an operating frequency which is a mechanical harmonic frequency of said crystal higher than its fundamental frequency, said crystal having relatively mechanically inactive surface portions, a thin film of conducting material evaporated on a surface of said crystal and forming an electrode, said thin film having a portion extending over one or .said inactive surface portions, an electric circuit terminal, means for clamping said terminal to said thin film over said inactive surface portion to maintain electric connection between said terminal and film, and; means for supporting said crystal from another of said inactive portions.

16. In combination, a piezoelectric crystal, a pair of electrodes each partially covering an electrode face of said crystal, an uncovered area of each of said crystal faces being in register, re-

spectively, with a covered area of the other of said wherein said electrodes are integral. with the members secured to said crystal adjacent the said non-covered areas of its said electrode faces, each of said-supports contacting one of said electrodes and comprising an electrical connection thereto.

19. In combination, a piezoelectric crystal, a

pair of electrodes secured respectively to the op posite electrode faces of said crystal, said electrode faces having non-covered areas adjacent diagonally opposite edge portiono of said crystal,

and means for supporting said crystal at its said diagonally opposite non-covered areas.

20. In combination, a piezoelectric crystal, a pair of evaporated-metal electrodes partially covering'the electrode faces of said crystal, noncovered areas of said electrode faces being adjacent opposite ends of said crystal, and a pair of supporting members secured to said crystal adjacent the said non-covered areas of its electrode faces, each of said supports contacting one of said electrodes and comprising an electrical connection thereto. V

21. In combination, a piezoelectric crystal having opposite faces each partly covered with a another corner of said crystal element, conductive supporting means mounting said crystal element at said corners and establishingindividual electrical connections with said coatings at said corners, said means comprising conductive spring wires. V

24. A thickness-mode piezoelectric crystal element having substantially rectangular opposite major faces, the thickness of said crystal element between said opposite major faces being made of a value corresponding to the value of said thickness-mode frequency thereof, a pair of conductive coatings formed integral with said metal film, the metal covered areas of said crysarea of the other face.

22. A thickness-mode piezoelectric crystal element having substantially rectangular opposite opposite major faces, said conductive coatings being disposed opposite each other at the'central portions only of said major faces and formingelectric field-producing electrodes spaced entirely inwardly of all ofthe peripheral edges of said major faces, one of' said coatings on one of said major faces extending to one corner of said crystal element and the other of said coatings on the other of said major faces extending to another corner of said crystal element, conductive supporting means mounting said crystal entirely inwardly of all of the peripheral edges major faces, the thickness of said crystal element between said opposite major faces being made of a value corresponding to the value of said thick-' ness-mode frequency thereof; a pair of conductive coatings formed integral with said opposite major faces, said conductive coatings being disposed opposite each other at the central portions only of saidmajor faces and forming electric field-producing electrodes spaced entirely inof said major facesj and a pair of relatively narrow connective conductive coatings formed integral with said opposite major faces and extending from said field-producing conductive ductive coatings each partially covering an electrode surface of said crystal element, the uncovered area of each of said crystal electrode surfaces being opposite a covered area of the ners and establishing individual electrical 'connections with said coatings at said corners, said means comprising conductive spring wires;

23. A thickness-mode piezoelectric crystal element having substantially rectangular opposite major faces, the thickness of said crystal element between said opposite major faces being made of a value corresponding to the valueof said thickness-mode frequency thereof, a pair of con-- inwardly ofnall of the peripheral edges of said major faces, one of saidccatings one of said major faces extending to'one corner of said crystal element and the other of saidcoatings on the other of said mllor faces other of said crystal electrode surfaces, and the covered areas of each of said crystal electrode surfaces being disposed opposite each other only at the central portions of said crystal electrode surfaca said oppositely disposed covered areas forming opposite field-producing electrodes having an effective field area covering lessthan per cent of the area of one of said crystal electrode surfaces, said field area being spaced entirely away from all of theperipheral 01' marginal edges of said crystal electrode surfaces.-

27. In combination, a thickness-mode piezo- 'of each of said crystal electrode surfaces being opposite a covered area of the other of said crysstal electrode surfaces, and the-covered areas of each of said crystal electrode surfaces being disposed opposite each other only at the central porextending to 76 tions of said crystal electrode surfaces said oppositely disposed covered areas forming opposite field-producing electrodes spaced entirely away from all of the peripheral or marginal edges of said crystal electrode surfaces, said oppositely disposed covered areas forming opposite field-producing electrodes of substantially circular shape having an efiective field area less than to per cent of the area or one of said crystal electrode surfaces. r

28. In combination, a piezoelectric crystal ele-- ment havin its thickness made of a value corresponding to the value of its thiclmess mode frequency, and a pair of conductive coatings each partially covering an opposite major snrface of said crystal element, each of said coatings covering a marginal edge portion oi one of said major surfaces and tapering in width from said i marginal edge portion thereof towards th cen- F. BA MVVIN. 

